Relationship between altered axial curvature and neural tube closure in normal and mutant (curly tail) mouse embryos

Neural tube defects, including spina bifida, develop in the curly tail mutant mouse as a result of delayed closure of the posterior neuropore at 10.5 days of gestation. Affected embryos are characterized by increased ventral curvature of the caudal region. To determine whether closure of the neuropore could be affected by this angle of curvature, we experimentally enhanced the curvature of non-mutant embryos. The amnion was opened in 9.5 day embryos; after 20 h of culture, a proportion of the embryos exhibited a tightly wrapped amnion with enhanced curvature of the caudal region compared with the control embryos in which the opened amnion remained inflated. Enhanced curvature correlated with a higher frequency of embryos with an open posterior neuropore, irrespective of developmental stage within the range, 27–32 somites. Thus, within this somite range, caudal curvature is a more accurate determinant for normal spinal neurulation than the exact somite stage. Enhanced ventral curvature of the curly tail embryo correlates with an abnormal growth difference between the neuroepithelium and ventral structures (the notochord and hindgut). We experimentally corrected this imbalance by culturing under conditions of mild hyperthermia and subsequently determined whether the angle of curvature would also be corrected. The mean angle of curvature and length of the posterior neuropore were both reduced in embryos cultured at 40.5°C by comparison with control embryos cultured at 38°C. We conclude that the sequence of morphogenetic events leading to spinal neural tube defects in curly tail embryos involves an imbalance of growth rates, which leads to enhanced ventral curvature that, in turn, leads to delayed closure of the posterior neuropore.